Ultra-high vacuum conditions, and apparatus capable of maintaining an ultra-high vacuum, are widely used to control environments requiring exceedingly low levels of contamination, such as the environments required for the fabrication processes of integrated circuit semiconductors, linear accelerators, surface analytical instrumentation and space simulation chambers. Ultra-high vacuum environments, below 10.sup.-9 torr, require that constituent chamber materials have intrinsically low vapor pressure. At room temperature, the combined efflux, due to outgassing of residual contamination and the vapor pressure of all of the materials used in the fabrication of the device, should not exceed the desired pressure. Furthermore, at moderately elevated temperatures, up to 400.degree. Celsius, the vapor pressure of the materials used in the construction of the chamber should not exceed the speed at which the vacuum pumps can evacuate the efflux from the vessel. Elevating the temperature while simultaneously vacuum pumping a container is referred to as "baking-out", or thermal cycling, and results in a dramatic reduction of contamination via evaporation by reduction of pressure and removal of the source of outgassing. If the materials used in the fabrication of the vacuum device are also intrinsically impermeable to the passage of contamination, the vacuum levels may be maintained by appropriate pumping systems.
Vacuum systems of this type thus require near perfect seals which are able to withstand the thermal and pressure cycling of the system, and are also able to maintain a seal at all aperture points and openings to the environment outside of the vacuum system while subjected to a wide range of temperatures and pressures.
Particularly important are the seals provided for inlets through the chamber wall of an ultra-high vacuum or pressure vessel for performance of specialized functions within the vacuum chamber. For instance, to provide inlets for light into a vacuum or pressurized system, hermetically sealed optical elements are required.
Presently, viewports which are suitable for ultra-high vacuum use are manufactured using windows metallized on the outer perimeter and brazed or soldered to a supporting metal member using a suitable vacuum alloy, or by fusing the viewport material directly to a thinned-metal member such as a glass-metal seal or Housekeeper(tm). These methods are limited to use principally with three optical materials: sapphire (crystalline aluminum oxide), fused quartz (amorphous silicon dioxide) and sealing glass (amorphous borosilicates). The resulting ultra-high vacuum viewports produced by these metallization methods are typically not of high optical quality because of strains induced in the optical material by the sealing process.
Furthermore, materials compatible with seals of this type are limited by several factors. First, the metallization process requires that the substrate material have an oxygen atom as part of its chemical makeup. Second, the materials joined by brazing or fusing are subjected to extremely high temperatures, greater than 800.degree. Celsius, to set-up the seals prior to cooling to room temperature. Even if the optical material has an adequately low vapor pressure to survive such excursions in temperature, it must also be capable of withstanding the resultant sealing stresses induced by dissimilar expansion rates of the materials. Partial compensation for different materials may be achieved by thinning the metal member and carefully selecting the metal components to ensure expansion characteristics similar to the optical substrate. Third, the direct fusion of an optical element to a supporting metal member requires that the optical material be wet to adhere to the metal member through the growth of an oxide layer therebetween.
These above described metallization methods then constitute permanent methods of manufacturing hermetically sealed optical assemblies generally suitable for viewing, but not suitable for producing distortion free optical assemblies. Additionally, these assemblies are not compatible with many materials which transmit outside the visible and near infrared region of the electromagnetic spectrum.
Alternate methods of constructing vacuum optical assemblies exist which are not appropriate for ultra-high vacuum use. These methods usually employ materials which are either permeable to gaseous contamination or have an intrinsically high vapor pressure. Seals of this type are typically produced at room temperature or at low temperatures. These methods include elastomeric O-ring seals, epoxy seals, silver halide seals. O-ring seals employ materials which are not compatible with thermal cycling and are to a certain extent permeable to gases. Epoxy materials outgas due to their high vapor pressure. Silver halide seals are permeable to gases, are not compatible with high temperature excursions, and further require a fluorine constituent in the optical material.
Finally, mechanical joints may be made to optical elements using wire or foil gaskets. Seals are created by causing the gasket material to flow, thereby filling imperfections and gaps between the metal and the optical substrate. The metal must be deformed beyond its elastic limit to flow into the imperfections; therefore, this method requires either extremely low elastic strength metals, such as indium or lead, or an optical material strong enough to withstand the large loads necessary to deform the gasket material. Indium and lead melt at low temperature. Silver, copper, aluminum and gold all have tensile strengths which vary generally from 10,000 psi to greater that 60,000 psi. Because sapphire and quartz may be joined to metal by brazing, no increase in the number or type of optical materials is gained by this method.
Joints employing foil or wire seals generally have relatively stiff and massive metal members intentionally selected for rigidity to produce a deformation in the gasket material when the sealing load is applied. The alignment between the optical element and sealing surfaces must be extremely flat, and maintained during thermal cycles, to withstand the high sealing load. Typically, the metal component selected has expansion characteristics nearly matching the expansion characteristics of the optical substrate. Misalignment between the rigid metal components and the optical element may cause a gap to develop, or the optical element may break because of concentrated stress at "point-stresses". These hermetically sealed optical assemblies intrinsically have high stress thereby producing unwanted strain in the optical material. Frequently, such strain is catastrophic, resulting in fracture of all but the strongest, useful optical materials. Even without fracture, the strain may be sufficient to produce optical distortions.
A heretofore unmet need exists for optical assemblies enabling low-strain ultra-high vacuum seals to be formed at room temperature to optical elements such as windows, lenses, prisms and the like. Particularly needed are optical assemblies using fragile materials, requiring low distortion, employing special coatings incompatible with thermal cycling, and utilizing materials having chemistries incompatible with metallization or glass-to-metal fusion techniques.